The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for stabilizing control pressure in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulley in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of an engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (upshaft). At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed rate which is decided by the line the pressure, pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio of the transmission.
Japanese Patent Laid Open No. 62-52261 (U.S. Pat. No. 4,674,363) discloses a hydraulic circuit provided with a reducing valve so as to generate a constant control oil pressure, for example, of 4 kg/cm.sup.2. The oil pressure is supplied to the transmission ratio control valve and line pressure control valve as a basic oil pressure. Two solenoid operated valves are provided to regulate the constant pressure so as to apply appropriate control pressures to the transmission ratio control valve and line pressure control valve.
In the hydraulic circuit, in order to firmly hold the belt on the pulleys at each transmission ratio, line pressure varies from 5 to 30 kg/cm.sup.2 dependent on engine torque. On the other hand, the pressure controlled by the reducing valve must always be constant, for example at 4 kg/cm.sup.2, despite the change of the line pressure. However, the solenoid operated valves are adapted to be closed when the transmission ratio is large and the engine load is heavy, and to open when the transmission ratio is small and the engine load is light. Thus, the amount of oil supplied from the oil pump through the reducing valve is maximum at the smallest transmission ratio and minimum at the largest transmission ratio. Therefore, when the line pressure is high, the amount of oil drained from the reducing valve is large. Accordingly, the oil pump must have a large capacity. If there is not enough , the line pressure is reduced, thereby causing the belt to slip on the pulleys.